The present invention relates to the recovery of precious metals such as gold and silver from aqueous solutions thereof. Generally, the precious metals, e.g., gold are present in the form of cyanide complexes such as potassium or sodium gold cyanide. Such cyanide solutions are obtained or are the by-product of processes such as gold plating, leaching of ores containing precious metals, and stripping of gold or other precious metals from waste material. Although the present invention is particularly concerned with a precipitating agent, and method, for recovering gold from aqueous alkali metal gold cyanide solutions, it is not limited thereto. The problem dealt with by the present invention is generally the recovery of precious metals such as gold, silver, platinum, palladium and rhodium from aqueous cyanide solutions thereof by precipitating the precious metals as elemental metal.
The prior art has devoted attention to this task. For example, U.S. Pat. No. 3,271,135 discloses that the use of zinc dust to precipitate gold from gold cyanide complex solutions is known. This patent proposes as an alternate solution the use of an alkali metal hydrosulfite or hydrazine reducing agent, and an aldehyde group-containing compound to precipitate metallic gold.
U.S. Pat. No. 3,271,136 notes that one difficulty encountered was the tendency of the precipitated gold to redissolve in the solution. This is apparently due to the build-up of alkali metal cyanides in the solution as the gold (or other precious metal) is precipitated from the cyanide complex. This redissolution is sometimes described as a "yo-yo" effect, as precipitated gold is redissolved.
An earlier patent, U.S. Pat. No. 1,426,517 discloses the use of platinum or palladium metal to enhance the ability of hydrazine to reduce nickel compounds in suspension or solution to finely divided elemental nickel suitable for use as a catalyst.
In addition to causing problems of precious metal (e.g., gold) redissolution, the presence of cyanide in the solution after treatment creates problems of waste disposal, since the cyanide compounds are, of course, highly poisonous.
It is accordingly an object of the present invention to provide a novel and efficient precipitating agent for precipitating gold and other precious metals from aqueous alkaline solutions thereof, including aqueous alkaline cyanide solutions.
It is another object of the invention to provide a precipitating agent which, in addition to precipitating the precious metal from solution, destroys a portion at least of the cyanide content, if any, of the solution thereby alleviating problems both of precious metal redissolution and treatment of cyanide waste for disposal.
It is another object of the invention to provide a novel and efficient precipitating agent which contains aluminum powder and which provides recovered gold of extremely high purity.
It is also an object of the present invention to provide a novel method of precipitating precious metals from alkaline aqueous cyanide solutions, and reducing the cyanide content thereof, which method employs the precipitating agent of the invention.
It is also an object of the present invention to provide a novel method of precipitating precious metals from alkaline aqueous solutions, which method employs the precipitating agent of the invention.
Other objects and advantages will appear from the following description.